Vermiculite is a sheet aluminosilicate of the hydromica group having a monoclinic crystal system. In the nature vermiculite is formed by weathering or a hydrothermal process at the expense of mica. A simplified general chemical formula of vermiculite can be expressed as (M+,M2+)0.45-0.75AM(Y2+,Y3+)5-6O [(Al, Si)8 O20]T(OH)4T8xH2OA, in which formula the structural sites are expressed with letters A, O and T. A denotes an interlayer, O an octahedral-fold coordinated site and T a structure-forming tetrahedral-coordinated site. Furthermore, in the general formula M+ denotes principally a K+, Na+, or NH4+ cation, and M2+ denotes a cation, typically Mg2+. Sometimes minor amounts of Ca2+ and Ba2+ cations may be present in A-site. Mg2+ and Fe2+ occupy Y2+-positions, Fe3+ and Al3+ occupy Y3+-positions.
In vermiculite's structure O- and T-sites follow each other in proportion 1:2, and they form a sandwich-like structure with an interlayer between each two single T-OT-successions. A schematic presentation of vermiculite's layered structure is presented in FIG. 1.
Isomorphous substitutions take usually place in all structural units of vermiculite. The dimension of the interlayer in C-direction, conventionally measured as basal spacing distance Cd002, varies dependent on the cation occupancy in the interlayer. Depending on the interplane distance, different cations have a preference for occupying the interlayer of the vermiculite structure. Magnesium ions normally occupying most of the interlayer sites can be substituted to e.g. potassium, sodium, rubidium, calcium, barium or ammonium ions. Vacancies can also be present in the crystal structure. Hydration-dehydration reactions can take place in the interlayer.
Water layers are also available in the interlayer site of the vermiculite structure. Pseudohexagonal rings in the tetrahedral-fold site are coordinated with OH-groups. During heating to a temperature up to about 1100° C. the vermiculite is dehydrated and exfoliated. During the heating, crude vermiculite passes through five discrete structural transformations, caused by the step-wise dehydration of vermiculite. Each dehydration step corresponds to the shrinking of the lattice dimensions of the vermiculite crystal, e.g. measurable as Cd002 The first three steps of dehydration are reversible, whereby the second and third dehydration steps take place at temperature ranges of 190-280° C. and 300-440° C., respectively. The last two dehydration/dehydroxylation steps are irreversible, leading to consequent decomposition of the vermiculite to talc and, further, to enstatite Mg2Si2O6. During the first three dehydration steps H2O is released from the lattice, but during the last two steps OH is released. The reversibly dehydrated vermiculite can be hydrated back in an aqueous solution, and the compressed lattice is getting expanded back to the initial state.
It is known that vermiculite has a capacity to incorporate ammonium into its structure. However, commercially available expanded vermiculites and vermiculite bearing products are not efficient enough in their ammonium intake so that they could be used commercially for removal of ammonium ions from the environment.
In the nature ammonium ions function as a nutrient. Therefore removal of excess of ammonium ions e.g. from waste water is important before water is fed back to the environment in order to avoid eutrophication. It would be beneficial to have an absorption material that would be able to absorb fast relatively large amounts of ammonium ions. It would also be an advantage if this absorption material could be used further after ammonium absorption, instead of taking it to the waste tip.